Growers watch fields for resistant weeds

Bob Scott and Chris Tingle | Feb 14, 2003

Approximately 75 percent of the soybeans grown in Arkansas are Roundup Ready. This is around 2.5 million acres. The percentages of acres that are Roundup Ready in other states, have not risen to quite this level.

The reason for the large number of acres in Arkansas is the genuine need for the technology. Palmer amaranth and common cocklebur long ago developed resistance to herbicides such as Scepter and Classic in the state. Arkansas also has documented populations of Prowl/Treflan-resistant pigweeds and goosegrass.

Couple that with a long growing season, plenty of rain or irrigation, crop rotational issues and a large soil bank of weed seeds and you have a perfect fit for using Roundup Ready soybeans and glyphosate.

However, considering the history of herbicide resistance, it seems only a matter of time until the Roundup technology meets a fate similar to herbicides that have come (and gone) before.

Herbicide resistance builds up over time when the same herbicide is used again and again to control weeds. It works a lot like natural selection. You kill all the susceptible weeds and somewhere out there in the field a lone plant survives. This plant has a unique enzyme system or some other trait that allows it to withstand the herbicide application.

It goes to seed. Some of its progeny have the same trait. When the same herbicide with the same mode of action is used again, it is not effective on these plants. With continued use of this product, the resistant population continues to build and eventually takes over the field. At this point the herbicide that used to work so well is no good, at least on that weed.

With other modes of action this process has taken as little as seven years. So the question keeps coming up at grower meetings this spring — “When will we get resistance to Roundup?” You might already have it.

Several species of weeds have already been documented with glyphosate resistance. In Australia, some ryegrass has been found to be resistant to glyphosate and several other herbicide families and Roundup-resistant goosegrass has been documented in the Northeastern United States.

Probably the most famous weed in the Delta right now is glyphosate-resistant marestail or horseweed. In Tennessee where no-till cotton is grown, resistant horseweed populations have grown significantly over the last three or four years.

A few suspicious fields have been tested and observed in Arkansas, but there have been no documented cases yet.

The 2.5 million acres of Arkansas Roundup Ready soybeans get treated one or two times with glyphosate in crop. Some possibly get a burndown application of glyphosate followed by two in-crop glyphosate treatments. In addition, we have conventional burndown acres, Roundup Ready cotton, and Roundup Ready corn which all receive at least one application of glyphosate.

It would be safe to say that over 4 million acres are treated with glyphosate annually in Arkansas. To say the least, this is a tremendous amount of selection pressure. In the past, it was believed that due to the complex enzyme system that glyphosate interacts with, the buildup of resistance would be unlikely. However, the recent development of resistant weeds in the field and subsequent lab studies show that resistance can happen.

Also, there are theories that “weed shifts” and not resistance will be the first real challenge for the soybean technology. This is similar to the shift in perennial weeds that is observed when fields go from conventional tillage to no-till. The bottom line is that no matter how resistance develops, once you have it you need new technology.

The most disturbing thing for soybean weed control is that companies are not looking very hard for new soybean herbicides right now. If widespread resistance were to happen today, we would have to rely on old chemistry coming back into play.

What can you do to prevent resistant weeds from building up on your farm? The most obvious thing is rotate chemistry. In other words, do not use glyphosate for a season or at least for one of your applications. I suggest using paraquat (Gramoxone) as a burndown in place of Roundup, for example.

However, you have to make sure that the herbicide that you switch to is not the same mode of action. For example, switching from Roundup to Touchdown would not be changing your mode of action. You would still be putting the same selection pressure on your farm.

Another option is going back to conventional soybeans. Although the yield gap is now almost gone, there are still some very high-yielding conventional soybean varieties out there and the cost of many conventional herbicides has gone down.

Many conventional herbicides are still not as economical as glyphosate, but compared to what you used to pay, they are cheaper.

Many herbicide resistance strategies involve simply tank-mixing glyphosate with another herbicide. This is fine for the management of a specific weed problem. However, in the grand scheme of things, you are still applying the same selection pressure on your farm again and again. So tank-mixing is better than nothing, but still promotes resistance.

Roundup Ready soybeans and Roundup herbicide offer the most economical and broad-spectrum weed control ever available for soybean farmers. In all probability, we will continue to use this technology as much as possible until resistance does develop.

It is unlikely that widespread rotation of chemistry and crops will be utilized to prevent the buildup of resistant weeds. In the words of several growers, “It is just too good and too cheap not to use.”

One thing we can do is to watch our fields very closely and report any suspicious weeds that survive one or more applications of glyphosate. Prior to any re-treatment, these weeds should be sampled and studied. If you suspect that a weed or weed population has survived a glyphosate application that in the past has controlled that weed, please contact your county agent.

Bob Scott is the University of Arkansas Extension weed specialist. [email protected]. Chris Tingle is the University of Arkansas Extension soybean agronomist.